This invention relates to a method of producing soda lime silica glass sheet products bearing a thin, functional, coating of tin oxide to promote reflectivity of infrared radiation. These coatings are also electrically conductive. Generally, when a skilled artisan is talking about the ability of a coating to reflect infrared radiation. he uses the term emissivity. The lower the value of emissivity, the better the tin oxide coating is in reflecting infrared radiation.
Glass having an infrared radiation reflective coating thereon has substantial value in the construction industry. Such a coating is useful in providing windows with enhanced insulating value because they reflect infrared radiation back into the interior of the building in which they are located. Infrared radiation is a product of the fuel burned in the building. If the infrared radiation is allowed to escape through the window, more fuel will be required in order to keep the building at the same temperature. However, when windows having an infrared reflective coating thereon are employed, much of the infrared radiation is reflected back into the interior of the building. The lower the emissivity value of the coating on the windows, the more infrared radiation is reflected by the coating and returned to the interior of the building. Thus, a minor improvement in the emissivity of the coating is of major concern because such improvement means that the same coating will reflect more infrared radiation back into the interior of the building.
A search was conducted on the subject matter of this specification in the United States Patent and Trademark Office. The search resulted in the citation of the following U.S. Pat. Nos.: 2,566,346; 3,107,177; 3,677,814; 3,959,565; 4,146,657 and its reissue Re. 31,708; 4,235,945; 4,265,974 and 4,500,567. Each of these patents will be discussed individually below. None of the patents are felt to teach or suggest the method of my invention for the reasons set forth below.
U.S. Pat. No. 2,566,346 issued Sept. 4, 1951 for Electroconductive Products And Production Thereof. This patent teaches a method of providing a glass base with an electroconductive coating. The method is carried out by heating the base to a temperature above about 400.degree. F. but below the temperature at which the glass becomes molten. Thereafter, an aqueous solution containing a tin compound and an ionizable fluoride is applied to the hot base. The patent does not disclose a pretreatment with a fluorine-containing compound as is taught in my method.
U S. Pat. No. 3,107,177 issued on Oct. 15. 1963 for a Method Of Applying an Electroconductive Tin Oxide Film And Composition Therefor. This patent teaches a method of producing a transparent electroconductive tin oxide film having a haze factor less than one percent. The tin oxide film is also capable of intimate electrical contact with a ceramic silver bus bar on a surface of a ceramic base. The method is carried out by heating the surface of a film to a film forming temperature and applying to the heated surface a composition having a pH between 4.5 and 7. The composition also contains an organic tin compound, HCl, and an ionizable fluorine-containing compound. The HCl concentration is up to four percent by weight based on the weight of ten. The surface of the ceramic base is heated to a film forming temperature above 400.degree. F. at which the HCl component provides lower resistivity to the electroconductive film than said film forming composition consisting of the organic tin compound and an ionizable fluorine-containing compound as the essential ingredients. The method of this patent does not disclose a fluorine-containing compound be used for pretreating the surface of the ceramic material.
U.S. Pat. No. 3,677,814 issued on July 18, 1972 for a process For Forming Electroconductive Tin Oxide Films By Pyrolyzation Of Alkyl And Aryl Tin Fluorides. This patent teaches the formation of transparent electroconductive tin oxide films by pyrolysis of a tin organic compound which has a direct tin-fluoride bond. The patent does not disclose a pretreatment of the glass substrate with a fluorine-containing compound prior to the application of a tin oxide coating thereon.
U.S. Pat. No. 3,959,565 issued on May 25, 1977 for a Tin Oxide Coating. This patent disclosed a method of coating a sheet of glass with tin oxide by intermittent spraying of a mixture of materials in a non-aqueous solution and in an oxidizing atmosphere. The glass is maintained at a fixed temperature by supporting the glass in molten liquid at a controlled temperature. The spraying of the glass is conducted in successive passes involving small amount of spray per pass. The glass is moved continuously along a pool of the molten liquid during the spraying process. This patent does not teach the initial application of a fluorine-containing compound to the surface of the glass sheet prior to the application of the doped tin oxide film.
U.S. Pat. No. 4,146,657, subsequently reissued as Re. 31,708, issued on Mar. 27, 1979 for a Method Of Depositing Electrically Conductive Infra-red Reflective Transparent Coatings Of Stannic Oxide. This patent discloses electrically conductive films of tin oxide prepared by a process utilizing gaseous chemical compounds. These compounds react to form a tin-fluorine bond at a temperature which is (1) high enough so that the newly-created tin-fluorine bond-bearing molecule remains in the vapor phase, and (2) low enough so that oxidation of the molecule occurs only after the indicated rearrangement. Films prepared by the disclosed process are characterized by surface resistance as low as 1 ohm per square when the film thickness is as thin as about a micron. The patent does not disclose pretreatment of the glass with a fluorine-containing compound prior to the application of the doped tin oxide film.
U.S. Pat. No. 4,235,945 issued on Nov. 25, 1980 for High Resistivity Electroconductive Tin Oxide Films. The patent discloses a method for pyrolytically forming electroconductive tin oxide films having higher resistivity for a given thickness than tin oxide films formed by known methods. The method is carried out by adding to the film-forming solution an organic compound which retards the rate of film formation reducing the particle size of the tin oxide film thus increasing the resistivity of the film. Small quantities of an organic compound such as cresol or benzyl alcohol are added to the film-forming solution. The patent does not teach the pretreatment of a glass sheet by a fluorine-containing compound prior to the application of a doped tin oxide film.
U.S. Pat. No. 4,265,974 issued on May 5, 1981 for an Electrically Conductive, Infrared Reflective, Transparent Coatings Of Stannic Oxide. This patent has a disclosure similar to the aforementioned U.S. Pat. No. 4,146,657 and no further discussion will be undertaken herein. It also has the same deficiencies as the aforementioned patent.
U.S. Pat. No. 4,500,567 issued on Feb. 19, 1985 for a Method For Forming Tin Oxide Coating. This patent teaches a method for forming a fluorine-containing tin oxide coating on the surface of a substrate. The substrate is contacted after being heated with a vapor of monobutyltin trichloride or a mixed vapor of monobutyltin trichloride and a doping agent composed of a fluorine-containing compound of the following formula XCHF.sub.2 wherein X represents a fluorine atom, a chlorine atom, or a hydrocarbon group consisting only of carbon and hydrogen atoms. This patent does not teach the application initially of a fluorine-containing compound to a heated glass sheet prior to the application thereto of a doped tin oxide film.